1. Field of the Invention
This invention relates to a process for desmutting aluminum and aluminum alloy surfaces, especially those of high silicon aluminum alloys, by contacting the surfaces with a particular aqueous liquid composition.
2. Statement of Related Art
Common chemical and even mechanical treatments of aluminum often leave the surface with a dark coating that must be removed before subsequent surface finishing steps can be satisfactorily completed. This process is generally known in the art as "desmutting" or sometimes as "deoxidizing". Current commercial desmutting practice normally uses oxidizing acid solutions containing some form of chromium(VI). This material, of course, has severe pollution potential, so that an alternative, equally effective desmutting composition and/or process would be highly desirable.
Some desmutting compositions without chromium have previously been known in the art. For example, "Research Disclosure" 273,037 according to an abstract thereof teaches desmutting in concentrated nitric acid; Japanese Laid-Open Patent Application 59-1,699 according to an abstract thereof teaches desmutting in a mixture of nitric and hydrochloric acids; published British patent application GB 1,399,111 according to an abstract thereof teaches desmutting with a solution of ammonium persulfate, sodium bisulfate, and ammonium nitrate; U.S. Pat. No. 3,634,262 according to an abstract thereof teaches desmutting in a solution containing alkali, alkaline earth, or ammonium peroxydisulfate(s); acid salts of sulfuric acid; and, optionally, fluorides; U.S. Pat. No. 3,647,698 according to an abstract thereof teaches desmutting with a solution of urea nitrate and ferric sulfate, optionally also including boric acid and/or fluoride ions; and U.S. Pat. No. 3,510,430 according to an abstract thereof teaches desmutting with a solution of ferric sulfate, alkali metal bisulfate, alkali metal nitrate, and alkali metal silicofluoride. None of these teachings is believed to have achieved substantial commercial success.